1. Field of the Invention
The present invention relates to a liquid ejection head and an image forming apparatus, and more particularly, to a liquid ejection head and an image forming apparatus in which liquid is supplied to a plurality of pressure chambers from a common flow channel which accumulates the liquid, and the liquid inside the pressure chambers is pressurized by pressure generating devices and is ejected from nozzles connected to the pressure chambers.
2. Description of the Related Art
In the related art, a print head based on an inkjet method (inkjet head) is known, in which ink is supplied from a common flow channel, which accumulates the ink, to a plurality of pressure chambers, the ink inside the pressure chambers is pressurized by pressure generating devices, typically piezoelectric elements, and the ink is thereby ejected from nozzles connected to the pressure chambers.
In order to prevent fluid crosstalk in the print head, it is common to use a structure having air dampers provided inside the print head. The fluid crosstalk is a phenomenon where the acoustic waves generated by a particular pressure chamber during ink ejection travel through the common flow channel and penetrate into the other pressure chambers, thereby having an adverse effect on ink ejection in the other nozzles. The main cause of the fluid crosstalk is acoustic waves of 200 kHz to 300 kHz, which is the resonance frequency of the pressure chamber system. Japanese Patent Application Publication No. 2003-127363, for example, discloses a structure in which air dampers are provided inside the common flow channel so that the acoustic waves are attenuated by means of the deformation of these air dampers to prevent the fluid crosstalk.
However, there are problems in the print heads in the related art as follows.
An index of the performance of the air damper is the acoustic capacitance Cd, which represents the deformability of the damper. A damper having a high acoustic capacitance Cd is excellent at preventing the fluid crosstalk. When the damper is taken to be a beam structure with two fixed ends, the damper's acoustic capacitance Cd is directly proportional to the fifth power of the damper width W, and inversely proportional to the third power of the damper thickness t. In other words, increasing the damper width W and reducing the damper thickness t serve to raise the damper's acoustic capacitance Cd. However, in recent years, with the increasing density of nozzle arrangements, it has become impossible to achieve sufficient space for installing air dampers, and the width W of dampers has become narrowed. Consequently, a situation arises in which sufficient acoustic capacitance Cd cannot be guaranteed in the damper, and hence it becomes impossible to prevent the fluid crosstalk. Of course, it is possible to raise the damper's acoustic capacitance Cd by reducing the thickness t of the damper. However, if the damper's thickness t is reduced, then the damper becomes more liable to fracturing and there is an increased risk of ink leakage. In this way, there are problems in methods which use the air dampers, and from a cost viewpoint, it is desirable to be able to prevent the fluid crosstalk by means of another method, other than the air dampers.
As a method other than one based on the air dampers, a composition may be adopted in which the acoustic waves generated when ink is ejected are attenuated inside the common flow channel. However, it is not possible to expect attenuation of the acoustic waves inside the common flow channel. This is because the acoustic attenuation constant in ink (liquid) is low, and furthermore, because the common flow channel has a small size and the acoustic waves only travel a short distance therein, generally in the order of several millimeters. In other words, the acoustic waves are hardly attenuated at all, and hence the fluid crosstalk cannot be prevented.
Therefore, it has been thought that rather than seeking to attenuate the acoustic waves inside the common flow channel, a composition should be adopted which allows the acoustic waves to escape into the main body of the head (for example, into a wall defining the common flow channel, and the like). The main body of the head is solid and has a greater damping effect than the ink (liquid), and furthermore it has a length of several tens millimeters, and hence the acoustic waves travel through a relatively long distance. Consequently, the acoustic waves are attenuated more readily than they are in the ink (liquid). However, in practice, it is difficult to get the acoustic waves inside the common flow channel to escape into the main body of the head. This is because there is extremely high acoustic reflectivity between a liquid and a solid.
Here, as shown in FIG. 15, a case is considered in which an acoustic wave propagated through the ink inside the common flow channel strikes the main body of the head (solid) perpendicularly. Taking Z1 to be the acoustic impedance of the ink (liquid), and Z3 to be the acoustic impedance of the head main body (solid), the acoustic energy transmission rate T0 is expressed in the following formula (1):
                              T          0                =                              4                                          (                                                                                                    Z                        1                                                                    Z                        3                                                                              +                                                                                    Z                        3                                                                    Z                        1                                                                                            )                            2                                .                                    (        1        )            
For example, if Z1=1.5×106 (Ns/m3) (the acoustic impedance of the liquid), and Z3=45.7×106 (Ns/m3) (the acoustic impedance of SUS 347 stainless steel), then according to the formula (1), the acoustic energy transmission rate T0=13%. In other words, a little under 90% of the energy of the acoustic wave is reflected by the head main body, and this reflected wave generates crosstalk.